In a stratified charge spark ignited engine, the air/fuel mixture is deliberately prepared in a non-homogeneous fashion to produce a small kernel of relatively rich mixture at the sparkplug while the remainder of the mixture is relatively lean. In one type of stratified charge engine, on the intake stroke, a large amount of very lean mixture is drawn through a main intake valve to a main combustion chamber. At the same time a small amount of rich mixture is drawn through an auxiliary intake valve into a pre-combustion chamber. At the end of the compression stroke, a sparkplug ignites the rich mixture in the pre-combustion chamber. As the rich mixture ignites, it in turn ignites the lean mixture in the main chamber. In such engines, therefore, the air/fuel mixture can be burned completely even if the air/fuel mixture, as a whole, is lean. Accordingly, fuel economy can be improved and regulated gaseous emissions, for example CO, NO.sub.x etc. contained in the exhaust of the engine can be reduced.
Various designs exist which eliminate the need for a pre-combustion chamber. For example, in U.S. Pat. Nos. 5,050,557 and 5,237,974, a charge stratification is produced by providing a multiple intake port engine where a fuel injector is placed in the air flowing through one of the intake pores such that the rich fuel mixture exits at the outlet of that intake port. The sparkplug fires near that intake port thereby igniting the rich mixture which in turn ignites the lean mixture.
Further, it is known that combustion in an internal combustion engine can be improved by controlling the amount of turbulence within the combustion chamber. A typical method of introducing such turbulence is to create a tumbling charge motion within the cylinder. Tumble can be achieved in a number of ways including unique intake port designs having a triangular cross-section as disclosed in U.S. Pat. No. 5,237,974 or the use of a tumble control valve (TCV) as disclosed in U.S. Pat. No. 5,165,374. In engines employing a triangular intake port design, manufacturing issues arise in controlling the dimensions for the triangular cross-section as well as increased manufacturing cost in producing such a design. And, more importantly, wide-open throttle power is limited by the intake port design. In engines using TCVs, a stratified charge has not be obtained because the fuel is introduced upstream of the TCV, which results in undesirable air/fuel mixing and fuel build-up on the surface of the TCV that causes a spread of the fuel across the inlet port resulting in non-stratified (homogeneous) charge. Additionally, the precise transient fuel delivery required by the stratified charge engine is not obtainable. As used herein, "upstream" means at a location remote from the cylinder of the engine. Conversely, "downstream" means at a location near the cylinder of the engine.